Compton polarimetry revisited

Abstract

We compute the average polarisation asymmetry from the Klein-Nishina differential cross section on free electrons at rest. As expected from the expression for the asymmetry, the average asymmetry is found to decrease like the inverse of the incident photon energy asymptotically at high energy. We then compute a simple estimator of the polarisation fraction that makes optimal use of all the kinematic information present in an event final state, by the use of "moments" method, and we compare its statistical power to that of a simple fit of the azimuthal distribution. In contrast to polarimetry with pair creation, for which we obtained an improvement by a factor of larger than two in a previous work, here for Compton scattering the improvement is only of 10-20 %.Key words: Hard X-ray, gamma-ray, Compton scattering, polarimeter, polarisation asymmetry, optimal variable 1. Cosmic-source polarimetry: the high-energy frontier Polarimetry is a powerful diagnostic of specific phenomena at work in cosmic sources in the radio-wave and optical energy bands, but very few results are available at high photon energies: the only significant observation in the X-gamma energy range, to date, is the measurement of a linear polarisation fraction of P = 19 ± 1% of the 2.6 keV emission of the Crab nebula by a Bragg polarimeter on board . At higher energies, hard-X-ray and soft-gamma-ray telescopes that have flown to space in the past (COMPTEL

HARPO, TPC as a gamma telescope and polarimeter: First measurement in a polarised photon beam between 1.7 and 74 MeV

“…At lower photon energies, from radio-waves to soft-X-rays, polarimetry provides a powerful insight to phenomena at work inside cosmic sources, in particular it provides a means to measure the magnetic field. Above a couple of MeV, this diagnostic is missing: not only does the Compton cross section decrease at high energy, but its polarisation asymmetry also decreases like 1/E [1]. Polarimetry would allow, for example, to decipher leptonic from hadronic models of blazars [2].…”

“…In a homogeneous detector though, the resolution for the azimuthal angle varies like E 1/4 (eq. (18) of [8]): there is hope that in the lower part of the energy spectrum of a pair telescope, where most of the statistics lies, the dilution stays high 1 . We have written an event generator which samples from the full, 5D, exact down to threshold (without any high-energy approximation) differential cross section, and includes polarisation [8].…”

HARPO, a gas TPC active target for high-performance γ-ray astronomy; demonstration of the polarimetry of MeV γ-rays converting to Bernard1 2019Nuclear Instruments and Methods in Physics Research Section A: Self Cite10040View full textAdd to dashboardCite

“…For Compton polarimeters, both the total cross-section and the polarisation asymmetry decrease with energy ( Fig. 2 of [7]). For nuclear pair-conversion, γZ → e + e − Z, the multiple scattering undergone by the electron and by the positron in the tracker blur the azimuthal-angle information carried by the pair within ≈ 10 −3 radiation lengths [8,9,10].…”